Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically written areas to carry ticketing or financial information, however magnetically written data is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages” which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a “book”, each sheet can be individually addressed. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal material that ceases to present an image when de-energized.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Said structure has the capacity of maintaining one of the given states in the absence of an electric field.
U.S. Pat. No. 3,816,786 discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The conductors in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one conductor must be light absorbing and it is suggested that the light absorbing conductor be prepared from paints containing conductive material such as carbon.
U.S. Pat. No. 5,289,301 discusses forming a conductive layer over a liquid crystal coating to form a second conductor. The description of the preferred embodiment discloses Indium-Tin-Oxide (ITO) over a liquid crystal dispersion to create a transparent conductor.
Cholesteric materials require one of the two conductors to be light absorbing and conductive. Materials have been proposed for the application including carbon or metal oxides to create a black and conductive surface for polymer dispersed cholesteric liquid crystal materials. Such coatings often backscatter light. Moreover, because there is inactive material between the conductors, it would be desirable to maximize the use of the inactive material.
U.S. Pat. No. 6,639,637 describes an opaque field-spreading layer for dispersed liquid crystal coatings that allow the electrical switching of material in the usually inactive regions (gaps) between the second conductors. U.S. Pat. No. 6,707,517 describes a transparent field-spreading layer for dispersed liquid crystal coatings that allow the electrical switching of material in gaps between the first conductors. Although field-spreading layers and state switching of gap material was demonstrated, the sequence of signals necessary for switching the gaps in passive matrix displays into the most desired pattern of light and dark states was not recognized.
There are many means for driving cholesteric liquid crystals in a passive matrix. U.S. Pat. No. 5,644,330 teaches how to utilize the right slope of the electro optical curves of chiral nematic liquid crystal. In addition, a clearing voltage is described that initializes the panel's reflective state. This clearing voltage can be sufficient to set the display into the focal conic texture. If the clearing voltage is high enough, the panel can be initialized into the stable planar texture. This prior art teaches that clearing the display before rewriting it assists in the removal of residual previous image. These drive methods can be described as conventional focal conic rest, right-slope select and conventional planar rest, right-slope select methods respectively.
Another drive method for cholesteric liquid crystals is pulse cumulative, as described in U.S. Pat. No. 6,133,895. In this method, a series of voltage pulses are applied to the display at a frequency approximately 60 Hz. A series of 6 or 7 voltage pulses cumulatively change the reflectance state of the pixel in an array of pixels. Methods of this type can be characterized as pulse-accumulation right-slope select.
Dynamic drive methods are also well known in the art of driving chiral nematic liquid crystal displays. U.S. Pat. No. 5,748,277 describes a 3 phase dynamic drive method where the fast transition from homeotropic to the transient planar texture is leveraged for high speed chiral nematic liquid crystal writing. SID 2001 Digest, pp. 882-885 “Simple Drive Scheme for Bistable Cholesteric LCDs” (Rybalochka, et. al) teaches a simple dynamic drive method, also referred to commonly as a drive scheme, utilizing a pseudo 3-phase that requires only 2 voltage levels for row and column drivers. This method is known as the “U/√2” driving method.
U.S. Patent Application 2005/0024307 A1 discloses yet another driving scheme for chiral nematic liquid crystal displays that follow a more conventional approach. This paper documents the use of a high voltage planar reset pulse to the entire panel prior to writing the display, as well as use of the left slope of the electro-optic curve for cholesteric liquid crystal displays. This drive method applies a high voltage pulse followed by a low voltage series of pulses to specific pixels that are to be transitioned from the stable planar texture (established in the reset pulse) to focal conic. During the selection phase, the voltage across the pixels to be transitioned to the focal conic texture is greater than the voltage across the pixels that are to remain in the stable planar texture, which is the distinguishing characteristic of a left-slope drive method. Therefore, this scheme can be described as a planar reset, left-slope selection method.
It is the aim of this invention to describe displays having drive voltage-reducing layers that do not significantly spread the field laterally and therefore allow displays to be written with any previously disclosed drive method to their best advantage without disturbing the reflection state of the gaps.